New ideas on how to hunt for alien life

Column: Looking at hills and valleys on Earth, scientists spot clues on how life affects the land.

The morning sun reflects on the Gulf of Mexico and the Atlantic Ocean as seen from the Apollo 7 spacecraft during its 134th revolution of the Earth on October 20, 1968.

Exobiologists are looking for every conceivable sign of alien life and habitable planets. But there may be clues that few have thought of. Taylor Perron, a geophysicist at the Massachusetts Institute of Technology, thinks his research team has found one in the familiar, repeating pattern of ridges and valleys that shape many landscapes on Earth.

Their report, published July 23 in the journal Nature, shows that the activity of living creatures help sculpt their terrain. It helps establish the spacing between ridges and valleys.

"The ridge-valley wavelength is one way that Earth's landscapes bear the imprint of life," says Professor Perron.

He adds that he hopes such research will define other clues that make it possible to infer the presence or absence of life on Mars and other planets.

That's good news for exobiologists, the scientists that consider life on distant planets, as they plan to meet the challenge that the National Research Council has thrown at them. On July 7, the NRC included the search for life beyond Earth among its new strategic goals for the US space program. That raises the scientific importance of the exobiologists' quest to a new level.

Perron and his colleagues have taken advantage of new topographic maps generated by aircraft-borne laser altimeters. They found ridge-valley patterns in forested areas as well as in open country. It now is evident that most landscapes show these patterns. With the help of computer-based simulations, the scientists were able to identify the fundamental forces shaping the patterns and controlling their size.

Running water cuts a sharp ridge-valley profile. Soil slowly creeping down hill smooths this out. Critters burrowing into the soil encourage this down-slope creep. Spacing between valleys – the pattern wavelength – ranges from 30 meters to 300 meters in the United States. It correlates with rainfall and the hardness of base rock. The wetter the climate or the harder the rock, the wider the valleys grow. According to the MIT announcement, Perron suspects life modulates the rainfall effect. Soil creep would be stronger where rainfall is more abundant. Tree roots and rodent burrowing would encourage water to travel underground and not erode the surface. Thus topography becomes a sign of life.

There's also a new way to assess the mineralogy of some distant planets. Astronomers at the University of California at Los Angeles find it in some white dwarf stars.

White dwarfs are the remnants of dead stars. In several cases, the UCLA researchers find unexpected materials in the stars' outer layer. This suggests the dwarf has swallowed an external object. For several years, the researchers thought it might have been an asteroid left over from the former star system. Now they say that the white dwarf GD-362 probably swallowed a planet. That means that the polluting material they see in GD-362 may provide a catalogue of the mineralogy of that planet and, by extension, of any similar, perhaps life-friendly, planets in other star systems.

With insights like those reported here, exobiologists are ready to boldly go where no biologist has gone before.

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